Apple didn't move or change the antenna so nothing has been fixed since the launch about a year ago. When holding it, avoid touching the antenna or get a case/ skin that covers this part up (which will stop your skin conducting the signal).

Is it everywhere, or only in some locations?
Have you changed networks when you got the iPhone 4, or is it the same? Did your previous phone perform better in the same location on the same network? How good is your reception? Can you browse heavy website with Safari without wanting to pull your hair out?
Can you compare it to a different handset

Apple didn't move or change the antenna so nothing has been fixed since the launch about a year ago. When holding it, avoid touching the antenna or get a case/ skin that covers this part up (which will stop your skin conducting the signal).